This invention relates to a low power liquid conductivity. measuring circuit for use in an oceanographic probe or portable salinometer.
The salinity of seawater is related to conductivity and it is well known that salinity can be measured in this way. There are various techniques and apparatus for measuring conductivity of seawater by sensing apparatus lowered into the sea on the end of a cable. There is also apparatus available for use at the surface on ships and in land based laboratories for measuring the conductivity of water samples taken in sample bottles by oceanographic survey organization and other groups interested in the salinity distribution of seawater in the various bodies of water throughout the world.
Earlier conductivity measuring circuits are described in U.S. Pat. No. 3,474,330 issued Oct. 21, 1969 to applicant; U.S. Pat. No. 3,495,164 issued Oct. 10, 1970 to applicant; U.S. Pat. No. 3,757,205 issued Sept. 4, 1973 to applicant; and in U.S. Pat. No. 3,963,979 issued June 15, 1976 to applicant. In addition, salinometer conductivity cells and measuring circuitry therefore are described in U.S. patent application Ser. No. 416,013 filed Sept. 8, 1982 now U.S. Pat. No. 4,511,845 issued April 1985 in the names of applicant and Christopher G. M. Kirby.
It is an object of the invention to provide a very low power conductivity circuit having a minimum number and cost of components which can be packaged in small size for use in an oceanographic probe or portable salinometer.
It is a further object of the invention to provide a circuit that can cope with substantial resistances and polarization voltages at the current and potential electrodes of the measuring cell and allow for the fact that the cell is a flowthrough tube open and grounded at both ends to the ocean in the case of the in-situ cell or grounded through unknown inpedances at both ends in the case of the salinometer.
It is a further object of the invention to provide a circuit designed to ensure nearly zero dc current in the conductivity cell to prevent generation of gas at the current electrodes and have provisions for at least partial balancing of the circuit so that the smallest significant variation can be easily amplified to easily measured values.
It is a further object of the invention to provide a circuit that will give measurements at frequent intervals and in short periods of time so that other measurements can also be taken while the sensor probe is being traversed through the water, for instance by lowering from a ship.